Broad band nonreflective neutral density filter

ABSTRACT

A broad band low reflectance neutral density optical filter. A neutral density filter having substantially flat transmittance and low reflectivity over a wide range of wavelengths from the ultraviolet through the visible region of the spectrum includes a transparent substrate, two successive layers of different dielectric materials on the substrate, at least two layers of metallic material, each metallic layer being separated from the preceding metallic layer by a layer of dielectric material, and finally, two succeeding layers of different dielectric mateials. The resultant neutral density filter thus has two layers of different dielectric materials between the substrate and the first layer of metallic material, and two layers of different dielectric materials between the atmosphere and the final layer of metallic material. This construction provides physical and spectral stability, relatively constant transmittance as a function of wavelength, and low reflectivity from both sides of the filter as a function of wavelength, which in turn permits a number of filters of the invention to be employed in series.

FIELD OF THE INVENTION

This application relates to optical filters, and more particularly, tobroad band low reflectance neutral density optical filters for use inthe ultraviolet and visible regions of the spectrum.

BACKGROUND OF THE INVENTION

Neutral density filters to be employed in the visible (VIS) region ofthe spectrum are commonly constructed of absorbing filter glasses. Suchmaterials are not useful in the ultraviolet (UV) spectral region between200 and 350 nm, however, since they have strong absorption bands atthese wavelengths.

Neutral density filters for use in the UV region are generally metal ormetal alloy films on quartz substrates. INCONEL 600 (TM) is the metallicalloy typically selected for such films, because it closely approximatesideal neutrality. Other materials can also serve the same purpose.

A simple film of metal on a transparent substrate functions as anoptical filter throughout the UV and VIS spectral ranges, but attenuateslight by reflection as well as by absorption, the amount of reflectiontypically being 4% to 40% from each surface. When two or more suchoptical filters are placed in series to effect a higher value of opticaldensity, reflections between them cause the resulting optical density tobe significantly different from that predicted from Beer's Law. It isdifficult to calculate the correct resultant optical density without adetailed knowledge of the reflectance of each surface. Such knowledge isseldom available.

An additional difficulty with simple metallic films is that theygenerally carry no protective coatings, and thus are not spectrallystable since the outer metal layer oxidizes and some of this oxide isremoved upon cleaning.

The combination of high reflectivity and low spectral stability inherentin simple thin metallic film optical filters reduces their utility forcalibrations or other critical work substantially.

Placing a layer of a dielectric material at the surface of the metalfilm can provide antireflection properties as well as physicalprotection and resultant spectral stability. However, such a layer doesnot necessarily reduce the reflectivity of the filter over thewavelength range of interest sufficiently to permit the filter to beused in critical and/or multiple-filter applications. In addition, thisapproach generally distorts the neutrality of the filter even attransmission values as high as 60%. For filters of lower transmittancethe spectral neutrality of the filter is degraded substantially and suchan antireflective coating is effective over only a relatively smallwavelength interval.

In the paragraphs below, a number of structures having light-filteringproperties are discussed to illustrate the state of the art. Not all ofthese devices are optical filters, as the films were designed for otherpurposes.

U.S. Pat. No. 3,781,089 of Fay and Cicotta discloses a neutral densityfilter with reduced surface reflection, for use in the visible portionof the spectrum with photographic apparatus. The filter is formed ofalternating layers of a metal or metal alloy and a dielectric on atransparent substrate. The initial and final layers are of the metal ormetal alloy. The dielectric layer is stated to have a lower index ofrefraction than the metal or metal alloy layer, and is preferablysilicon monoxide, while the metal is preferably INCONEL(TM). Thisneutral density filter contains at least five layers, the number ofmetal or metal alloy layers being one more than the number of dielectriclayers. The thickness of the layers is selected to achieve apredetermined optical density, and the thickness of each of the layersof the dielectric is selected to reduce the reflectance of the precedinglayers of the filter to a minimum.

The Fay and Cicotta filter design is suitable for use in the visibleregion of the spectrum, but is unsuitable for critical use in theultraviolet region since its reflectivity is too high and variessubstantially as a function of wavelength. This neutral density filteralso is not expected to be spectrally stable since it possesses noprotective outer layer. Furthermore, as the Fay and Cicotta patentrequires a minimum of three layers of metallic material, this design isappropriate only for filters operating at or above certain opticaldensities. The first metal layer is of a thickness to produce an opticaldensity of approximately 0.25, and the additional metal layers raisethis number. The highest percent transmittance achievable with thisdesign is thus about 55%.

U.S. Pat. No. 3,990,784 of Gelber discloses an architectural glasscoated with two layers of metal separated by a layer of a dielectricmaterial. The ratio of the thicknesses of the two metal layers is aconstant, while the thicknesses of the metal layers are adjusted tocontrol the transmission properties of the coating. The thickness of thedielectric layer is such that the reflection from the coating is notstrongly colored in the visible range. The coating is provided with anantireflection surface, located either adjacent to the substrate or onthe outermost layer, facing away from the substrate. An additional layerof dielectric material can be provided to introduce color as seen fromthe exterior of the architectural glass without changing the lowreflection from the architectural glass on the inside of the building.In the event that the multilayer coating is on the outside of the glass,this additional layer of dielectric is located on the outer surface ofthe glass coating. As shown in FIG. 4 of the reference, when themultilayer coating is to be employed on the inward-facing side of theglass, two additional layers of dielectric material are employed, one onthe inward-facing surface of the coating and another between the coatingand the glass substrate, this latter material being for the purpose ofdetermining the color as seen from the outside. The inward-facing layerof dielectric provides antireflection properties. The first and lastlayers of dielectric in the coating are different materials, the formerhaving a high index of refraction and the latter having a low index ofrefraction. The dielectric spacer layer between the two metal layersalso has a high index of refraction, for best color as seen from theoutside.

This structure will not function as a low reflection broad band neutraldensity filter since both its transmittance and its reflectivity varysubstantially as a function of wavelength, and the reflectivity isobjectionably high at certain wavelengths in the UV and in the visibleranges.

U.S. Pat. No. 4,101,200 of Daxinger discloses a light transmitting andabsorbing multilayer coating for a transparent substrate, the initiallayer which contacts the substrate being of silicon dioxide, and theremaining layers alternating between chrome and silicon dioxide. Thiscomposition is spectrally unstable since the outer layer is a chromiumlayer which is subject to oxidation and loss of material upon cleaning.Additionally, transmission in the UV varies strongly as a function ofwavelength, making it unsuitable as a broad-band neutral density filter.

In view of the limited useful wavelength ranges and limited spectralstabilities of prior art neutral density filters, it would be verydesirable to have broad range neutral density optical filters covering awide range of optical densities, having variations in nominaltransmittance no greater than approximately 5% and reflectances nogreater than approximately 5% over a substantial portion of the combinedUV and VIS spectral range, and having protective outer layers to providegood spectral stability. Such filters are the subject of the presentapplication.

SUMMARY OF THE INVENTION

The present invention provides a neutral density filter having lowreflectance from both sides of the filter over a wide range ofwavelengths from the ultraviolet through the visible region of thespectrum, and having a substantially flat transmittance profile as afunction of wavelength. The filters of the invention typically vary inreflectance and transmittance by a maximum of ±5% over a wide range ofwavelengths.

The broad band neutral density optical filter of the invention is amultilayer device including a transparent substrate; a layer of a firstdielectric material on this substrate; a layer of a second dielectricmaterial on the underlying layer of first dielectric material; aplurality of layers of metallic material, the first of these layers ofmetallic material being located on the underlying layer of seconddielectric material; at least one spacing layer of dielectric material,one spacing layer of dielectric being located between successive layersof metallic material; a capping layer of dielectric material on the lastof the layers of metallic material; and a terminating layer of adielectric material different from the capping layer of dielectric andlocated on the capping layer. The final two layers of differentdielectric materials provide physical protection and spectral stability,as well as excellent broad band low reflectance characteristics from thefront side of the filter. The two layers of different dielectricmaterials between the substrate and the first layer of metallic materialprovide excellent broad band antireflection characteristics from theback side of the filter. Filters of desired optical densities areprepared by making appropriate selections of the numbers and thicknessesof the metallic layers of the filter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from a consideration of thefollowing detailed description taken in conjunction with the drawing, inwhich:

FIG. 1 is a cross sectional view of a broad band neutral density opticalfilter of the invention;

FIG. 2A is a chart illustrating the measured transmittance and frontreflectance of an 11-layer neutral density optical filter of theinvention having a nominal transmittance of approximately 12%;

FIG. 2B is a chart illustrating the rear reflectance of the filter ofFIG. 2A;

FIGS. 2C and 2D present the predicted optical properties of the neutraldensity filter whose actual transmittance and reflectance were presentedin FIGS. 2A and 2B;

FIGS. 3-11 are charts of the optical properties predicted for a seriesof filters of various nominal optical densities and numbers of metallayers, constructed in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, the neutral density filters of the inventioncomprise a transparent substrate 8, a layer 10 of a first dielectricmaterial coated on the substrate, a layer 12 of a second dielectricmaterial coated on the first dielectric layer 10, a first layer 14 ofmetal or metal alloy coated on layer 12 of second dielectric material, alayer 16 of dielectric material coated on metallic layer 14, a secondlayer 18 of metal or metal alloy coated on layer 16, possible furtheralternating layers of dielectric and metal or metal alloy, a cappinglayer 20 of dielectric on the final layer of metal or metal alloy, andfinally, a terminating layer 22 of another dielectric material.Dielectric material layers 10 and 12 are of different materials, as arethe final two dielectric layers 20 and 22. The layers of metal or metalalloy are generally of the same material, though layers of differentmetallic materials may in principle be employed. Similarly, the layersof dielectric material which are in contact with the layers of metal ormetal alloy are also generally composed of the same dielectric material,though this is not necessarily required.

Substrate 8 is transparent over the wavelength range of interest.Depending on this wavelength range, the substrate may be made of a widevariety of materials, including glass for the visible (VIS) range,quartz for the ultraviolet (UV) and VIS ranges, sapphire for the vacuumUV range of approximately 1700-2000Å certain plastics such as acrylicsand polycarbonates for the VIS range, and certain inorganic crystalssuch as magnesium fluoride for use in the far UV from approximately1200-2000Å. The transmission characteristics of possible substratematerials are generally known and in any case are readily measurable.For filters usable in the UV range above approximately 2000Å preferredsubstrate material is quartz.

The layers of metal or metal alloy employed in the neutral densityfilters of the invention are generally of nickel, chromium, or alloys ofthese metals such as nichrome and chromel. The absorption of radiationby these materials as a function of wavelength is relatively constant.The family of alloys containing nickel, chromium, and iron known asINCONEL (TM) is preferred, INCONEL 600 being especially preferred. Thethicknesses selected for the metallic layers are a function of theoptical density desired for the neutral density filter and also of theindex of refraction of the adjacent dielectric material. The number ofmetallic layers and their thicknesses determine the resultant opticaldensity. The thickness of each metallic layer is at least 10Å, with themaximum layer thickness being approximately 500Å. For practicalpurposes, the layers of metallic material are generally betweenapproximately 10 and 100 A thick. Between two and eight layers ofmetallic material are employed in the neutral density filters of theinvention.

The neutral density filters of the invention contain between five andeleven layers of dielectric materials, the thicknesses of these layersdepending in large part on the wavelength region over which the filtersare to be rendered relatively nonreflective. Dielectric films forfilters intended for use in the UV are typically 150-600Å thick, whilefilms for use in the VIS range are typically somewhat thicker. Thedielectric materials have indices of refraction in the range 1.3-2.5,preferred dielectric materials having refractive indices in the range1.35 to 2.1, the most preferred material for the 2000-7000Å wavelengthrange having an index of refraction of approximately 1.85. Å listing ofsome representative dielectric materials (and their nominal indices ofrefraction) is: aluminum oxide (1.63), quartz (1.46), silicon monoxide(1.55), titanium dioxide (2.20), magnesium fluoride (1.38), and hafniumoxide (1.85). Other dielectric metal oxides and fluorides will alsoserve, however.

The first dielectric layer 10 adjacent to the substrate has an index ofrefraction lower than that of the layer of second dielectric 12, and ispreferably of aluminum oxide. The dielectric layers in contact with theseveral metal or metal alloy layers are preferrably of hafnium oxide.Other dielectrics can also be employed, case-by-case optimization ofparameters being required. The outermost layer of dielectric materialhas an index of refraction different from the adjacent dielectricmaterial layer, and higher than that of the final metal layer, foroptimum achromaticity. This outermost layer is preferably of quartz, forfilters intended for use in the 2000-7000Å wavelength range.

In the neutral density filters of the invention substrate 8 providessupport for the several layers of dielectric material and metallicmaterial which comprise the optical filter. The layers of metal or metalalloy absorb a portion of the incident light, the amount of lightabsorbed being a function of the number and thicknesses of these layers.The layers of dielectric material which are adjacent to layers of metalor metal alloy render the adjacent metallic layers nonreflective, or asa minimum, reduce the reflectivity of the metallic layers to acceptablylow levels. The outermost two layers of dielectric material providedurability and scratch resistance, as well as excellent achromaticityfor the resultant optical filter. These two layers of dielectricmaterial, especially when employed in conjunction with an underlyinglayer of metallic material of an appropriate thickness, provide asignificantly improved bandwidth of low reflection, relative to a singlelayer of dielectric. Similarly, the combination of the two innermostlayers of dielectric material provide a broader nonreflective bandwidththan a single layer of dielectric, thereby providing excellent opticalproperties from the substrate side of the filter.

The parameters of a filter of the invention are determined by aniterative process, beginning with a defined filter configuration inwhich the numbers, types, and thicknesses of the layers comprising thefilter are selected arbitrarily. The indices of refraction of thedielectrics are obtained from the literature, and the optical constantsfor the layers of metal or metal alloy are either obtained from theliterature or are experimentally determined in a manner known to theart. With this information, transmission and reflectance curves for thedefined filter are calculated in a manner known to the art. In thisregard, see H. A. Macleod, "Thin Film Optical Filters," MacmillanPublishing Co., 1986. The necessary calculations are preferably carriedout with the aid of a suitable computer program such as "Film Star"interactive software sold by FTG Software, PO Box 358, Chatham N.J.07928, or other similar programs.

The approximate desired transmission level for the filter is nextpreliminarily determined by varying the numbers and thicknesses of thelayers of metallic material and recomputing the transmission curve aftereach adjustment, until a satisfactory value is reached.

Next, the flatness of this transmission curve over the wavelength rangeof interest is improved, if necessary, by making further adjustments tothe thicknesses of the metallic and dielectric layers of the filter,recomputing the transmission curve after each change. This optimizationmay also be done by a suitable computer program, once the layers whichare to be varied are specified.

Once a reasonably flat transmission curve is obtained over thewavelength range of interest, the initial reflectance curve for thefilter is calculated in a manner known to the art, preferably by aid ofa suitable computer program. Typically, the same computer program whichcalculates the transmission curves also calculates reflectance uponcommand.

The initially-calculated reflectance will usually be above 10% atcertain wavelengths, and commonly varies as a function of wavelength. Itis therefore necessary to optimize the reflectance curve to achieve aslow and as flat a reflectance over the selected range of wavelengths aspossible. The reflectance curve is improved, if necessary, by varyingthe thicknesses of the outermost and innermost two or three layers ofdielectric material, to optimize the reflectance from the front and rearof the filter, respectively, recomputing the reflectance curve aftereach change. Other dielectric layers may also be varied if desired, butsuch changes are generally not required. This optimization may also bedone by a suitable computer program, once the layers which are to bevaried are specified.

The aforementioned optimizations of the reflectances from the front andrear of the filter frequently result in changes in the predictedtransmission curve, necessitating reoptimization of the transmissioncharacteristics of the filter in the manner described above. In thisreoptimization of transmission, the thicknesses of inner metallic layersare preferably varied, to minimize disruption of thepreviously-determined reflectance characteristics of the filter.

The reoptimization of transmission may in turn change the predictedreflectance curve, necessitating its reoptimization. This process ofalternately optimizing the transmission and reflectance curves of thefilter is continued until the desired transmission and low reflectanceare achieved over the wavelength range of interest, and the respectivecurves are sufficiently flat.

The neutral density filters of the invention are prepared usingprocedures and equipment well known to those skilled in the art.Suitably-cleaned substrate blanks are placed in the substrate fixture ofan optical coating vacuum chamber equipped with sources of the severalcoating materials to be deposited, means for exciting these coatingmaterials sequentially into the vapor state, and means for monitoringthe thicknesses of the deposited coatings. The coating materials areexcited by an electron beam or by such alternatives as thermal orsputtering techniques. The monitoring means may be any of a variety ofdevices, such as a simple optical monitor with or without a chipchanger, or a calibrated thickness monitor such as a quartz crystalmonitor. The chamber is pumped down to a vacuum of approximately 6×10⁻⁵Torr or whatever vacuum is appropriate for the materials to bedeposited, an electric field is imposed across the apparatus to create aglow discharge in the chamber to provide final cleaning of the surfacesto be coated, then the chamber is repumped. Each coating material isheated in sequence at a sufficient temperature to vaporize it, and for atime sufficient to deposit the desired coating thickness.

EXAMPLE 1

A neutral density filter made up of successive layers of aluminum oxide(520A), hafmium oxide (220A), INCONEL (30A), hafmium oxide (285A),INCONEL (60A), hafmium oxide (370A), INCONEL (135A), hafmium oxide(330A), INCONEL (60A), hafmium oxide (186A), and quartz (525A) on aquartz substrate was prepared using the above-described procedure andequipment. Suitably-cleaned substrate blanks were placed in thesubstrate fixture of an optical coating vacuum chamber equipped withsources of the coating materials to be deposited, an electron beam forvaporizing the coating materials, and a quartz crystal monitor tocontrol the thicknesses of the deposited coatings. The chamber waspumped down to a vacuum of approximately 6×10⁻⁵ Torr, an electric fieldwas imposed across the apparatus to create a glow discharge in thechamber to provide final cleaning of the surfaces to be coated, then thechamber was repumped. Each coating material was heated in sequence at asufficient temperature to vaporize it, and for a time sufficient todeposit the specified coating thickness. The finished filter was thencharacterized with a spectrophotometer for reflectance and transmission.The transmission and reflectance curves for this filter are presented inFIGS. 2A and 2B, and those predicted for this filter are shown in FIG.2C. It is seen that the agreement between the predicted and experimentalcurves is quite good, though not perfect, and suggests that the chartsfor the predictive examples shown in FIGS. 3-11 are qualitatively andsemiquantitatively correct. The discrepancies are presumably due toslight errors in the optical constants employed in the calculations atcertain wavelengths, as well as slight errors in the thicknesses of thedeposited layers.

PREDICTIVE EXAMPLES

The following examples are provided to illustrate the opticalcharacteristics to be expected from neutral density filters constructedin accordance with the teachings of the present application.

The optical constants for a layer of metallic material at variouswavelengths are a function not only of the identity of the particularmaterial, but also depend on how the metallic layer was prepared. Thus,to obtain accurate optical constants at various wavelengths for layersof a given metallic material prepared by a given process, one mustprepare a layer of the material by the given process, take accuratemeasurements of reflectance from both surfaces, the transmittance, andthe physical thickness at a large number of wavelengths, and calculatethe requisite constants in the manner known to those skilled in the art.This is time-consuming and expensive, and as a result, not often done.Instead, researchers in this area frequently estimate the desiredoptical constants from data in the optical literature or employ thenumbers supplied by the manufacturer of the metallic coating material.This approach is generally adequate for qualitative or semiquantitativepurposes, but results in imperfect agreement between calculated andexperimental transmittance and reflectance curves.

The expected optical performance data were generated by computer, basedon input of the number, thicknesses, and optical constants for thevarious layers of the metallic and dielectric materials employed.

In these examples, INCONEL 600 (TM) is taken as the metal alloy. Hafniumoxide, having an index of refraction of 1.85, is taken as the dielectricmaterial contacting the layers of the metal alloy. The initial layer ofdielectric material in contact with the substrate is taken as aluminumoxide having an index of refraction of 1.63, and the outermost layer ofdielectric material is taken as quartz having an index of refraction of1.46.

The curves shown in FIGS. 3-11, were generated using the opticalconstants for INCONEL 600 listed in Table I.

                  TABLE I                                                         ______________________________________                                        INCONEL 600 OPTICAL CONSTANTS                                                 Wavelength                                                                             (n, k)       Wavelength (n, k)                                       ______________________________________                                        2000 Å                                                                             (1.050, 2.400)                                                                             2500 Å (1.100, 2.300)                               3000 Å                                                                             (1.900, 1.860)                                                                             3500 Å (1.750, 2.050)                               4000 Å                                                                             (1.700, 2.300)                                                                             4500 Å (1.900, 2.470)                               5000 Å                                                                             (2.050, 2.700)                                                                             5500 Å (2.300, 2.800)                               6000 Å                                                                             (2.600, 2.900)                                                                             6500 Å (2.900, 3.000)                               7000 Å                                                                             (3.300, 3.000)                                                                             8000 Å (3.600, 3.000)                               ______________________________________                                    

A number of predictive examples are summarized in Table II below. Theexpected optical properties of filter samples 1-9 are shown in FIG.3-11, respectively.

Other embodiments of the invention will be apparent to those skilled inthe art from a consideration of this specification or practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with the true scope and spiritof the invention being indicated by the following claims.

                  TABLE II                                                        ______________________________________                                        Exemplary Neutral Density Filters                                                    Layer              Layer         Layer                                 Layer  Thick-    Layer    Thick- Layer  Thick-                                Material.sup.a                                                                       ness.sup.b                                                                              Material.sup.a                                                                         ness.sup.b                                                                           Material.sup.a                                                                       ness.sup.b                            ______________________________________                                        Sample No.   Sample No.    Sample No.                                         1            2             3                                                    D(1) 520         D(1)   520      D(1) 520                                   D      220       D        220    D      220                                   M       15       M         20    M       20                                   D      420       D        297    D      285                                   M       15       M         40    M       40                                   D      225       D        292    D      340                                   Q      700       M         30    M       60                                                    D        186    D      330                                                    Q        524    M       30                                                                    D      186                                                                    Q      525                                   ______________________________________                                        Sample No.   Sample No.    Sample No.                                         4            5             6                                                    D(1) 520         D(1)   520      D(1) 520                                   D      220       D        220    D      220                                   M       30       M         30    M       50                                   D      285       D        285    D      285                                   M       60       M         60    M      100                                   D      340       D        340    D      340                                   M       70       M        100    M      170                                   D      330       D        330    D      330                                   M       40       M         40    M       80                                   D      186       D        186    D      186                                   Q      525       Q        525    Q      525                                   ______________________________________                                        Sample No.   Sample No.    Sample No.                                         7            8             9                                                    D(1) 520         D(1)   520      D(1) 520                                   D      220       D        220    D      220                                   M       40       M         40    M       40                                   D      300       D        330    D      300                                   M       60       M         60    M       60                                   D      300       D        330    D      300                                   M      200       M        100    M      100                                   D      350       D        300    D      300                                   M      140       M        200    M      200                                   D      370       D        360    D      300                                   M       60       M        140    M      130                                   D      180       D        360    D      330                                   Q      500       M         60    M      140                                                    D        180    D      340                                                    Q        450    M       60                                                                    D      180                                                                    Q      500                                   ______________________________________                                         .sup.a D(1) is aluminum oxide (Al.sub.2 O.sub.3). D stands for HfO.sub.2.     M stands for INCONEL 600 and Q stands for quartz.                             .sup.b Layer thicknesses are shown in units of Angstroms.                

I claim:
 1. A neutral density optical filter, comprising:a transparentsubstrate; a layer of a first dielectric material on said substrate; alayer of a second dielectric material on said layer of first dielectricmaterial; a plurality of layers of metallic material, the first of saidlayers of metallic material being located on said layer of seconddielectric material; at least one spacing layer of dielectric material,each spacing layer of dielectric material being located betweensuccessive layers of said metallic material; a capping layer ofdielectric material on the last of said layers of metallic material; anda terminating layer of a dielectric material different from said cappinglayer of dielectric material and located on said capping layer.
 2. Theneutral density filter of claim 1 wherein said substrate is quartz. 3.The neutral density filter of claim 1 wherein said first dielectricmaterial is aluminum oxide.
 4. The neutral density filter of claim 1wherein said second dielectric material is hafnium oxide.
 5. The neutraldensity filter of claim 1 wherein said metallic material is selectedfrom the group consisting of nickel, chromium, and alloys thereof. 6.The neutral density filter of claim 5 wherein said metallic material isa nickel-chromium-iron alloy.
 7. The neutral density filter of claim 1wherein said spacing layers of dielectric material are hafnium oxide. 8.The neutral density filter of claim 1 wherein said capping layer ofdielectric material is hafnium oxide.
 9. The neutral density filter ofclaim 1 wherein said terminating layer of dielectric material is quartz.10. The neutral density filter of claim 1 wherein the number of layersof metallic material is between two and eight.